Imprinting is a process whereby a domestic chick comes to recognise a conspicuous object by being exposed to it. Experimentally, 14 visually naive chicks were presented with a compound training stimulus (TC) during two 1h periods. The TC was a rotating, internally illuminated visual object (Tvis), a red box or blue cylinder, and a simultaneously presented recording of a maternal call (MC). Action potential activity (spikes) was recorded from the intermediate medial mesopallium (IMM), a polysensory area in the chick forebrain, that is a store for the memory underlying imprinting (1). Through imprinting, the proportion of IMM neurons responding to the visual imprinting stimulus increases dramatically, and non-linearly (2). Here we examine the effects of training on IMM neuron responses to the TC, and to its separate components, the Tvis and the MC. Chicks were anaesthetised (0.12ml equithesin, i.p.) and a recording assembly was fitted to the skull, enabling the introduction of tungsten microelectrodes to the IMM, two in each hemisphere (2). The next day, after recovery from anaesthetic, the electrodes were advanced until spontaneous spikes were detected. Each chick was placed in a running wheel and recordings were made during five Tests of neuronal activity (2), though we consider here only T1 (-0.75h) before the start of training, and T5 (25h) after training (times are the mid-point of ~1h tests). Experiments were terminated after the 5th test. The activities of individual neurons (1-12 per electrode) were sorted (2) from the spikes captured at each electrode. Neurons were tested for significant responses (paired t-test, P<0.05) during serial 4s presentations (15-20) of the Tvis, the MC, and the TC (Tvis and MC presented together). Comparisons between classes of responsive neurons were made using binary logistic regression. Recordings were made from 230 neurons at T1, and 251 neurons at T5. The percentage of neurons responding to the TC declined across Tests (P<0.01) from T1 (60.4%) to T5 (52.0%). However, by separating those TC-responsive neurons which also responded to the Tvis (TC+Tvis neurons) from those unresponsive to the Tvis (TC-Tvis neurons), we found an increase in TC+Tvis neurons (9.1% to 27.2%, P<0.001) similar to that reported previously for Tvis-responsive neurons (2). The decline in TC-responsive neurons is accounted for by a reduction in TC-Tvis neurons (51.3% to 24.8%, P<0.001). Most of the TC-Tvis neurons (75.4% at T1, declining to 50.0% at T5, P<0.001) were also responsive separately to the MC. In seven untrained chicks 59.2% (142/240) responded to the MC at T1, whereas 31.9% (51/160) did so at T5 (P<0.001). These findings suggest that neuronal changes in the IMM associated with imprinting involve a decrimental process that is largely related to the auditory component of the training compound, and an incremental process related to its visual component.